1. Field of the Invention
This invention relates to a switching mechanism for disconnecting apparatus from a high voltage DC power supply including a switching mechanism for making ground connections. More particularly, it relates to a high voltage switching mechanism that can be directly mounted on a power supply housing. Still more particularly, it relates to an improved high voltage switching mechanism capable of receiving power from a single bushing and selectively allowing power to be applied to one or a pair of loads.
2. State of the Prior Art
It is known to be desirable to have a high power DC power supply for industrial uses. One well known application of such DC power supplies is to power electrostatic precipitators for pollution control. To supply such DC power, it has been common to utilize a step-up transformer powered from conventional power lines, to drive rectifier circuits. It is common to have a housing for the transformer/rectifier (T/R), and to have the T/R circuitry immersed in oil for enhanced cooling. The relatively high DC voltages, for example in range of 50,000 volts, also requires attention to electrical insulation requirements.
In addition to the T/R circuitry, it has been common to utilize a high voltage switch (HVS) for providing multiple selections of modes of operation for one or two loads driven simultaneously. It has been common to locate the HVS within the T/R housing, and for it to be also immersed in oil. The HVS has essentially provided for five sets of switch selection positions for use with two loads such as the fields of two precipitators. The HVS selections provide for the selections of grounding both loads, providing full wave rectified voltage to either output while grounding the other, providing full wave rectified voltage to both outputs, or providing half wave rectified voltage to both outputs. Various ones of these selections will be discussed further below.
The HVS provides selected output voltages on a pair of electrical conductors, each of which passes through an electrically insulative bushing in the T/R housing. Such bushings are commonly made of an electrically insulative material, such as porcelain, and have exposed electrically conductive tips for connecting the respectively associated electrical conductors utilized for driving the loads.
Though used for a relatively long period of time, the T/R circuit arrangement in conjunction with the HVS used for driving pairs of loads has several operational and use deficiencies. From a use standpoint, with the HVS immersed in oil within the T/R housing, an operator could not see the positioning of the switch contacts. Accordingly, an operator could not be certain of a grounded contact selection. For safety reasons, then, an operator must provide auxilary grounding connections when the load equipment is to be worked on.
Due to the use of two bushings, and the requirement for physical spacing to avoid arcing, the conduit structure is required to be unduly large.
Functionally the prior art arrangement for a DC power supply has problems. The most serious problem occurs when the HVS is in the selection position to select full wave to be applied to two precipitators. When the field of one precipitator arcs, which can occur from one or two to many times a minute, the stored energy can be discharged through the HVS and the T/R. Further, if an electrical imbalance exists between the two fields, discharge through the power supply can also occur. When the energy stored in the non-arcing field is discharged through the T/R bushings, the air core reactors, and the HVS, damage can occur and the life usefullness of the equipment can be shortened.
In earlier times when the operation of the precipitator was controlled by passive elements, it was felt to be desirable to have the half wave selection. The disadvantages of such a configuration were the saturation of the transformer due to unbalance in the fields, and the reduced energization of each field due to the fact that voltage was applied less than fifty percent of the time. With the advent of electronic controls, the half wave configuration is no longer necessary nor desirable.